1. Field of the Invention
The present invention relates to a method for producing optical fiber preform by a dehydrating and a sintering of a porous glass base material.
2. Description of the Related Art
A conventionally known method to manufacture an optical fiber preform for manufacturing a silica based optical fiber is a method in which a porous glass base material is manufactured using a VAD method, an OVD method, and the like, then a glass particle (soot) included in the porous glass base material is transparently vitrified by a dehydration step, thereby obtaining an optical fiber preform. As a dehydration agent to dehydrate the porous glass base material, chlorine (Cl2), thionyl chloride (SOCl2), carbon tetrachloride (CCl4), and the like are used, and is blended with inactive gas and provided within the dehydration-sintering device.
Conventionally, helium (He) gas has been predominantly used as inactive gas which is blended to the dehydration agent for the purpose of serving as a carrier or for the purpose of dilution, as indicated in Japanese Unexamined Patent Application, First Publication No. 2002-68770; Japanese Unexamined Patent Application, First Publication No. 2002-187733; and Japanese Unexamined Patent Application, First Publication No. 2003-183042. Further, Japanese Unexamined Patent Application, First Publication No. H11-1336 states that, since helium gas has a large gas diffusion coefficient and a large gas solubility coefficient in glass, and since it is less likely that helium gas remains inside the glass compared to nitrogen and the like, helium gas exhibits an effect in which the production of air bubble inside the glass is inhibited.
Helium gas is extremely expensive compared to argon (Ar) gas and nitrogen (N2) gas and the like. Therefore, considering the cost of manufacturing, it is not preferable to use helium gas as the inactive gas to be blended with a dehydration agent in the dehydration step.
However, when argon gas is used as the inactive gas to be blended with a dehydration agent, if the dehydration step is commenced after the furnace core tube containing porous gas is replaced with argon gas, there is a possibility that an inadequacy in dehydration will occur. This is because, argon gas has a low heat conductivity compared to helium gas, leading to a difficulty in heat to be conducted to a center portion of the porous glass base material, and therefore, the temperature of the entire porous glass particle laminated body cannot be elevated to an adequate desired temperature. Meanwhile, when the heating temperature is increased in order to increase the temperature of a central portion of the porous glass base material, the dehydration and vitrification of the surface of the porous glass base material is promoted excessively. Moreover, it becomes difficult for the dehydration agent to diffuse to the interior portion of the porous glass base material, thereby creating a cause of the inadequacy of dehydration.
It is possible to promote the dehydration of the central portion by increasing the amount of processing time, instead of increasing the heating temperature. However, there is a problem in that the productivity drops, and the manufacturing cost increases.
The present invention is made in light of these objectives. An object of the present invention is to provide a method for producing optical fiber preform, which allows a dehydration and a sintering of the porous glass base material to be performed at a lower cost compared to conventional methods, and allows a dehydration to be performed effectively.